API: extended puzzle API

[Siderite]

Right now, in order to get a puzzle, you need to provide an angle and a difficulty (see https://lichess.org/api#tag/Puzzles/operation/apiPuzzleNext ), where the angle is a single theme, motif or opening name and the difficulty is one of "easiest", "easier", "normal", "harder", "hardest".

I would like to be able to provide a list of comma separated theme names, as well as a proper rating filter (minimum and maximum value). This will allow users to customize their puzzle mix.

Thanks!

[ornicar]

Multiple theme selection won't work, due to the way puzzle paths are precomputed to reduce the load of selecting a puzzle.